Molecular orientational motions in liquid crystals: A study by Raman and infrared band-shape analysis

In this paper we report an extensive study of the relevant static and dynamic parameters connected to microscopic order and reorientational motion of single molecules in liquid-crystalline mesophases. In particular, we have studied the temperature dependence of the second- and fourth-order parameters 〈$P_2$〉 and 〈$P_4$〉, and of the spinning and tumbling diffusion coefficients $D_{?}^r$ and $D_{\perp }^r$ in the nematic phase of several mesogens; some smectic phases were studied also. These results were obtained using a combined theoretical-experimental method newly developed by us, which allows the simultaneous determination of both static and dynamic parameters from the study of Raman and infrared band shapes by Fourier-transform analysis. The results are discussed in the framework of reorientational diffusion processes in anisotropic potentials. Implications on the nature of the nematic-isotropic phase transition are also given.